Custom laser conductor linkage for integrated circuits

ABSTRACT

An integrated circuit includes a substrate with doped regions, a patterned polysilicon layer defining contacts and local interconnects, a submetal dielectric, a two-metal layer metal interconnect structure with an intermetal dielectric layer, and a passivation layer. Like the dielectric layers, the passivation layer is optically transparent, thick, and planarized. Because of this, laser energy can be directed through the passivation layer to fuse two conductors of the top metal layer without delaminating the passivation layer. In addition, laser energy is directed through the passivation layer and the intermetal dielectric to fuse pairs of conductors in the lower metal layer. Thus, the present invention provides for reliable convenient circuit modification without disturbing dielectric and exposing metal features to moisture and other contaminants.

BACKGROUND OF THE INVENTION

The present invention relates to integrated circuit technology and, moreparticularly, to a method for post-manufacturing custom-modification ofphysically modifying an integrated circuit. A major objective of thepresent invention is to provide a simple, economical, and reliablemethod of linking metal conductors of an integrated circuit.

Advances in integrated circuit manufacturing technology have allowedever increasing functionality to be implemented on a single device. Atypical integrated circuit includes: a silicon substrate with regionsdoped to control their conductivity type, field and gate oxides forelectrically isolating the substrate from overlaying conductors, one ormore polysilicon layers for forming contacts and local interconnects, asubmetal dielectric layer, a metal interconnect structure, and apassivation layer. The metal interconnect structure can have one or moremetal layers. Where there are plural metal layers, the layers areseparated by intermetal dielectric, usually at least as thick as theunderlying metal. In addition, the submetal and intermetal dielectricsare often planarized so that the next metal layer can be formed on aflat surface. The passivation layer tends to be thinner, since it is notrequired to isolate conductors, and not planarized, since it is not usedas a base for subsequently formed features.

As integrated circuit designs have become more complex, it has becomeincreasingly difficult to ensure that a design properly implements allits intended functions. Accordingly, prototypes must be built and testedbefore a commitment is made to a large volume run. However, small volumeprototype runs can be quite expensive and time consuming. If a designdefect is found in a prototype, it is desirable to verify the new designprior to starting over to make a new prototype.

One increasingly popular prototyping approach uses electrically (e.g.,"field", or "user") programmable devices. The functions performed bysuch devices are determined after manufacture by electricallyprogramming the device. If defects in the programming of a first deviceare discovered, a second device can be correctly programmed andsubstituted for the first device. Even more convenient are electricallyreprogrammable devices in which a defective design program can simply beoverwritten by a corrected program. However, the variations that can beimplemented by programming are limited by the circuit as designed intothe programmable device. If the circuitry of the programmable device isdefectively designed, the defect cannot in general be corrected byreprogramming the device or by substituting a nominally identical devicewith different programming. Thus, even with programmable andreprogrammable devices, the problem of correcting defectively designedhard circuitry remains.

In an "antifuse" approach to electrically programming a circuit, a largevoltage differential is applied across two adjacent conductors (on thesame or different layers) so that the intervening dielectric breaksdown, thus creating a link. A problem with the antifuse approach is thatthe resulting link has a relatively high impedance, so large currents,e.g., driver currents, cannot be handled. Furthermore, the antifuseapproach is limited to creating links, whereas it is desirable to beable to create reliable opens along existing conductors as well.

It is also possible to physically modify an integrated circuit to createan open. For example, a laser can be used to cut a conductor to createan open. However, the laser energy will disrupt the overlayingdielectric, impairing the predictability of the modification as well asthe long-term reliability performance of the circuit.

Focused ion beam systems have been used to make connections as well asbreak them. A focused, rasterized beam of high-energy ions, e.g.,gallium ions, can be used to sputter and remove dielectric over metallines. If a break is desired, the beam can be used to precisely cutthrough the metal. If a new connection is desired between thus exposedconductors, the metal itself is left undisturbed; metal-bearing, e.g.,tungsten carbonyl, gas is admitted into the vacuum chamber. The ion beamis scanned from one metal electrical node to the other. The ion beamlocally decomposes the metal-bearing gas adsorbed onto the surface,leaving a conductive trace between the metal electrical nodes. Thistechnique is very flexible, permitting connections even between nodesthat are on the same or different metal interconnect levels and disposedfar apart on the integrated circuit.

The main advantage of the focused ion beam approach is that it allowsflexible modification of an integrated circuit, both before and afterintegrated circuit manufacture is completed. The focused ion beamapproach is costly in that the equipment is expensive and requires ahigh degree of skill on the part of the operator. If circuit breaks arerequired in isolated lines, the laser cutting approach is most costeffective. However, as with laser cutting, the focused ion beam damagesintermetal and passivation dielectric, the effects of which can bedifficult to predict and control. Accordingly, devices so modified arebest limited to design verification purposes; newly designed andmanufactured devices are still required for end uses.

What is needed is an improved method of modifying integrated circuits.It should provide for making connections as well as breaking them. Yet,the method should be more economical and less destructive than thefocused ion beam method.

SUMMARY OF THE INVENTION

In accordance with the present invention, a laser beam is directedthrough an intact dielectric to link two metal conductors of anintegrated circuit. The minimum dielectric thickness above theconductors is greater than, and is preferably at least twice, themaximum thickness of the conductors in order to maintain mechanicalintegrity during the procedure. The conductors are preferably spaced adistance less than twice their maximum thickness.

The wavelength of the laser is selected so that it is absorbed muchbetter by the metal than by the dielectric material. For dielectrics andmetals commonly used in integrated circuits, visible light lasers ofsufficient power tend to be suitable. In practice, the method involvesplacing the integrated circuit (which may still be on a wafer or alreadymounted in a package) under a microscope. Using a target light toilluminate an area covering the two metal features to be linked and thenpulsing the laser. The selected laser power is sufficient to fuse themetals so that they flow into contact, but insufficient to delaminate orablate the overlaying dielectric.

The thickness of the overlaying dielectric is selected to provide thestrength required to resist stress caused by the laser and subsequentdeformation of the underlying metal. The required thickness isconventionally achieved in integrated circuits with multi-layer metalinterconnect structures, as long as the conductors to be linked are on ametal layer other than the top metal layer. Accordingly, the method canbe applied to many integrated circuits designed without the presentinvention in mind. However, a much greater range of electricalmodifications can be provided for if the integrated circuit is designedwith consideration given to subsequent laser linking of conductors.

Accordingly, an integrated circuit can be manufactured so that thepassivation layer over the top metal layer has a minimum thickness atleast as great, and preferably at least twice as great, as the metalconductor thickness. In addition, the passivation layer can beplanarized to optimize its optical transmission characteristics andimprove the efficiency of laser energy delivery to the conductors. Inthis case, the passivation layer can be similar to conventionalintermetal dielectric layers. However, the purpose of the structure isdifferent since the thickness is not required for electrical insulationor for forming structures thereon. The invention thus provides for thepre-linking integrated circuit structure, as well as such structures inwhich laser linking of conductors has taken place.

In addition, the metal patterns can be designed for optimal laserfusion. Accordingly, unoccluded corners can be provided for pairs ofconductors for optional connection. The mechanical stresses associatedwith such corners serve as relatively predictable material transfersites. In addition, corners have a greater reservoir of material forforming the link. Alternatively, conductors to be fused can be madewider than other conductors to provide that extra reservoir forconnection by this method.

The laser fusion method is less flexible than the focused ion beam inthat the metal features to be linked must be close together and on thesame metal layer. On the other hand, the equipment required for laserlinking can cost an order of magnitude less than the equipment requiredfor the focused ion beam method. Little training is required to operatethe laser and there is no vacuum requirement. In the laser linkageapproach, passivation and dielectric material is left in place;therefore exposure to moisture is minimized and no additionalpassivation steps are required to ensure long-term device reliability.

Because of these advantages, the laser fusion approach is practical notonly for prototype modification but also for field programmability. Infact, integrated circuits can be designed with certain conductorsadjacent to each other to permit their linking to implementpredetermined functions. This laser linking method can be used insteadof or in addition to other methods for modifying circuits as describedabove. These and other features and advantages of the invention areapparent from the description below, with reference to the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a integrated circuit in accordancewith the present invention and a system for modifying the integratedcircuit.

FIG. 3 is a flow chart of a method in accordance with the invention formaking the integrated circuit of FIG. 1.

FIG. 3 is a plan view of a pair of layers of an integrated circuitmodified in accordance with the present invention.

FIG. 4 is a more detailed schematic illustration of the system of FIG.1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a system A1 for modifying anintegrated circuit IC includes a laser 10 for fusing together adjacentconductors of the integrated circuit. Integrated circuit IC includes asubstrate 12, gate and field oxides 14, a polysilicon layer 16, asubmetal dielectric layer 18, a metal interconnect structure 20, and apassivation layer 22, as shown in FIG. 1. Metal interconnect structure20 includes a lower metal layer 24, an intermetal dielectric layer 26,and an upper metal layer 28.

Lower metal layer 22 includes four conductors 30, 32, 34, and 36. Inaccordance with the present invention, adjacent conductors 34 and 36 arebeing fused by laser beam LB from laser 10 to form a link 38. Adjacentconductors 30 and 32 are shown prior to fusion. Upper metal layer 28includes adjacent conductors 40 and 42, which have been laser fused inaccordance with the invention to form link 44.

Intermetal dielectric layer 26 conventionally includes three sublayers:a lower silicon dioxide sublayer 50, a spin-on glass (SOG) sublayer 52,and an upper silicon dioxide sublayer 54. Upper silicon dioxide sublayer54 provides electrical isolation between upper metal layer conductors,e.g., 40 and 42, and lower metal layer conductors, e.g., 30, 32, 34, and36. It is polished back to provide a flat surface on which to form uppermetal layer conductors, e.g., 40 and 42. SOG sublayer 52 fills ininterstices between lower metal layer conductors to provide a moreplanar surface on which to deposit upper silicon dioxide layer 54. Lowersilicon dioxide sublayer 50 provides for better adhesion and conformityof SOG sublayer 52. Lower metal dioxide sublayer 50 and SOG sublayer 52are etched back to expose lower metal layer conductors, e.g., 30, 32,34, and 36, prior to deposition of upper metal layer 26. Upper metallayer conductors, e.g., 40 and 42, are electrically connected to lowermetal layer conductors, e.g., 30, 32, 34, and 36 by metal vias (notshown) through intermetal dielectric layer 26.

Also in accordance with the present invention, passivation layer 22includes three sublayers, a lower silicon dioxide sublayer 60, a spin-onglass (SOG) sublayer 62, and an upper silicon dioxide sublayer 64. Lowersilicon dioxide sublayer 60 provides better adhesion and conformity forSOG sublayer 62. SOG sublayer 62 fills in interstices between conductorsof upper metal layer 28 to provide a smoother surface on which todeposit upper silicon dioxide layer 64. In addition, SOG sublayer 62contributes additional thickness over upper metal layer conductors.Upper silicon dioxide layer 64 provides the final passivation forintegrated circuit IC. It is polished flat to provide favorable opticalconditions for laser transmission in accordance with the presentinvention.

Passivation layer 22 differs from conventional passivation layers inbeing thick and planarized. A more typical passivation layer comprisesthin nonplanar sublayers of silicon dioxide and silicon nitride. Thepresent invention provides a thicker passivation layer to provide themechanical strength required to withstand stresses induced by laserfusion of upper metal layer conductors. The present invention providesfor a planar passivation layer to optimize energy transfer at theair/oxide interface at the top of passivation layer 22.

Thus, passivation layer 22 is more similar to intermetal dielectriclayer 26 than to a conventional passivation layer. Passivation layer 22differs from intermetal dielectric layer 26 in that lower silicondioxide sublayer 60 and SOG sublayer 62 are etched back only to theextent required to promote adhesion. In the case of intermetaldielectric layer 26, the etch back is required to ensure that metalconductors and vias do not come in contact with SOG, which can poisonthe metal. In the case of passivation layer 22, lower silicon dioxidesublayer 60 isolates upper metal layer conductors from SOG sublayer 62and no vias are formed through passivation layer 22 so poisoning is nota concern.

Substrate 12 is of silicon; various regions of silicon substrate 12 aredoped with impurities to define electrically active CMOS circuitelements, e.g., sources, drains, and channels. Polysilicon layer 16defines gates and local interconnects; these are respectivelyelectrically isolated from substrate 12 by gate and field oxides 14.Polysilicon layer 16 is electrically isolated from metal interconnectstructure 20 by submetal dielectric layer 18.

Submetal dielectric layer 18 comprises three sublayers, a lower silicondioxide sublayer 70, a reflowable glass planarizing sublayer 72, and anupper silicon dioxide sublayer 74. Upper silicon dioxide sublayer 74provides the primary isolation between polysilicon layer 16 and metalinterconnect structure 20. To provide a flat surface on which to formmetal interconnect structure 20, upper silicon dioxide sublayer 74 ismechanically polished to a flat surface.

Reflowable glass sublayer 72 is deposited and reflowed. The result is arelatively smooth base for upper silicon dioxide sublayer 74. Lowersilicon dioxide sublayer 70 serves to provide enhanced conformity andadhesion for reflowable glass sublayer 72. Note that submetal dielectriclayer 18 is similar to intermetal dielectric layer 26 except thatreflowed glass is used instead of SOG to fill in interstices betweenconductors. Lower metal layer conductors, e.g., 30, 32, 34, and 36, areconnected to active circuit elements of substrate 12 and to gatecontacts and local interconnects of polysilicon layer 16 by metal vias(not shown) through submetal dielectric 18.

A method M1 for forming links such as links 38 and 44 is flow charted inFIG. 2. It consists of forming an integrated circuit with "linkable"conductors to be optionally shorted disposed adjacent to each other onthe same metal layer, at step S1. The metal layer having the linkableconductors is then covered with dielectric thick enough to provide themechanical strength required to maintain dielectric integrity in spiteof the stresses imposed by laser linkage of the conductors, at step S2.Target illumination can be used to site across the width of bothconductors, at step S3. The target area 80 used for forming link 38 isshown in FIG. 3; likewise, a target area 82 is shown for use in forminga link between conductors 30 and 32. Laser 10 is then pulsed withsufficient power to fuse and link the conductors while leaving theoverlaying dielectric intact, at step S4 of FIG. 2.

Integrated circuit modification system A1 for practicing method M1comprises a microscope 110, an illumination subsystem 112, and a stage114, as shown in FIG. 4. Microscope 110 includes compound microscopeoptics 120, schematically represented by a single lens in FIG. 4, beamsplitting mirror 122 to deflect field illumination onto integratedcircuit IC, beam splitting mirror 124 to deflect target illumination andlaser energy to a selected target area of integrated circuit IC,shielding 126 to exclude unwanted light from the optical path ofmicroscope 110, and an eyepiece 128 to permit a human to view thatportion of integrated circuit IC within the field of view of microscope110. FIG. 3 represents that portion of lower metal layer 24 with thefield of view of microscope 110. Optics 120 is adjustable so that thearea of the portion of integrated circuit IC within the field of viewcan be selected.

Illumination subsystem 112 includes a field illumination source 130, atarget illumination source 132, laser 10, a beam splitting mirror 134,and an aperture 136. Field illumination source 130 provides light tomicroscope 110. Beam splitting mirror 122 deflects a portion of thefield illumination along the optical axis of microscope 110 tointegrated circuit IC. Some of the field illumination incident tointegrated circuit IC is reflected back along the optical path ofmicroscope 110. A portion of this reflection is transmitted by beamsplitting mirrors 124 and 122 through eyepiece 128, enhancing theviewability of integrated circuit IC.

Target illumination source 132 provides target illumination to delimitand highlight a region of integrated circuit IC to be fused by a laserpulse. A portion of the light from target illumination source 132 passesthrough beam splitting mirror 134 and is shaped by aperture 136 toprovide the desired target illumination cross section at integratedcircuit IC. The shaped target illumination beam is reflected toward atarget region, e.g., 80, of integrated circuit IC by beam splittingmirror 124. A portion of the target illumination reflected by the targetregion is transmitted through beam splitting mirrors 122 and 124 toeyepiece 128. The target illumination is bright compared to the fieldillumination so that the target region is highlighted relative to therest of integrated circuit IC in the field of view of microscope 110 inthe image presented at eyepiece 128 to a human viewer.

Aperture 136 is adjustable so that the area of the target illuminationcross section can be matched to the area into which laser energy is tobe delivered. Preferably, the width of the target illumination beam atintegrated circuit IC matches the combined (outer edge to outer edge)width of the conductors to be linked, as indicated by target areas 80and 82 in FIG. 3. Aperture 136 can be moved so that the targetillumination can be moved to different positions within the field ofview of microscope 110.

Laser beam LB from laser 10 is reflected by beam splitting mirror 134toward aperture 136. Aperture 136 shapes laser beam LB, just as itshapes the target illumination so that the area exposed to laser energymatches that delimited by the target illumination. The spatiallyfiltered laser beam LB is reflected toward the target region bybeam-splitting mirror 124. The laser energy fuses both conductors 34 and36 in the target region so that metal flows between them, forming link38. In the process, the intermetal dielectric layer 26 is stressed, butretains its integrity.

Stage 114 supports integrated circuit IC and provides for threedimensions of movement: control 140 provides for movement in an xdimension, control 142 provides for movement in a y dimension, andcontrol 144 provides for movement in a z dimension, orthogonal to theoptical axis of microscope 110. Microscope 110 also provides for threedimensions of movement relative to stage 110. Movement in the x and ydimensions is used to move the microscope field of view relative tointegrated circuit IC. The area subtended by the microscope field ofview can be determined by adjusting the magnification of microscope 110.Movement in the z dimension is used to provide appropriate clearance ofan integrated circuit to be prepared. For example, while integratedcircuit IC is still in wafer form, system A1 can be used to modify anintegrated circuit already mounted in a package. Microscope 110 andstage 114 can be separated by z dimension movement to clear the package.Preferably, microscope 110 provides a long focal length to focus laserenergy on a packaged integrated circuit.

One consideration in the design of integrated circuit IC is the stressinduced by absorbed laser energy within the overlaying dielectricmaterial. The heat generated by the laser and the deformation of theunderlying metal structure can impose mechanical stress in thedielectric. The dielectric must have sufficient mechanical strength toprevent delamination due to this stress. It is estimated that thedielectric thickness over a fused metal area should be at least twicethe prefused metal thickness to ensure dielectric integrity afterfusion.

Conventional integrated circuits employ a passivation layer over a topmetal layer. The passivation layer typically includes a silicon dioxidesublayer and a silicon nitride sublayer, and constitutes an effectiveshield against moisture and other contaminants. However, such apassivation layer tends to be no thicker than the underlying metallayer. Hence, there can be a risk of delamination if an attempt is madeto layer fuse conductors in the top metal layer. However, if there ismore than one metal layer, conductors below the top metal layer can befused with little risk of delamination.

To maximize post-manufacturing flexibility, step S2 of method M1, FIG.2, involves designing integrated circuit IC with thick passivation layer22 so that top metal layer 26 can be modified without riskingdelamination. In addition, passivation layer 22 is planarized to improveits optical transmission characteristics. In other words, theorthogonality of the target illumination and laser beam LB to the topsurface of integrated circuit IC can be achieved most readily with aplanar passivation layer 22.

Step S3 involves selecting the target area for fusion. This stepinvolves placing integrated circuit IC on stage 114. Stage 114 is thenmoved until the target area is within the field of view of microscope110. Aperture 136 is then set so that the desired target area ishighlighted by target illumination.

At step S4, laser 10 is pulsed so as to fuse the conductors in thetarget area. The laser power and the pulse duration are selected so thatsufficient energy is provided to fuse the conductors and so that theoverlaying dielectric is not unduly stressed. The precise power andduration should be calibrated for a particular laser and optical train(in the microscope). In general, however, the energy required todelaminate a sufficiently thick dielectric will also splatter the metalso that either a short is not formed or a short that forms is notreliable. Thus, selecting a laser power and a pulse duration for optimalfusing would generally avoid delamination. In the present case, laser 10delivers 1 millijoule for 5 μS to target area 80, which is about 5μsquare.

In integrated circuit IC, thick, planarized passivation layer 22 isoptimized for practicing the present invention. Alternatively, anon-planarized passivation layer can be used. Other embodiments use athin passivation layer, such as a convention oxide/nitride combination.In this case, it is preferable to limit the method to fusing conductorsin metal layers below the top metal layer.

While in the preferred embodiment, only links are formed, the presentinvention can be used to create opens in conductors. The target area canbe centered on a single conductor in a region of an integrated circuitin which the distance of the nearest other conductor on the same layeris at least twice the conductor thickness. The laser energy should bethe minimum required to create the open to maintain dielectricintegrity. This can be accomplished by using the power suitable forforming links; using only as many pulses as required to create the open.

While the present invention has clear advantages over known methods ofmodifying integrated circuits, it has its own limitations inflexibility. However, when combined with other methods, it provides agreater degree of modification flexibility than has been heretoforeachieved. For example, links between layers can be achieved using theanti-fuse approach; but the resulting connection has lesscurrent-carrying capacity than the laser link approach of the presentinvention.

In addition, the present invention can be combined with focused ion beamtechniques so that remote links can be formed. In this case, the laserlink approach can reduce the number of connections that required the FIBtechnique, or laser linking can be used in conjunction with focused ionbeam milling for device modification. Finally, the invention furtherprovides for a variety of stages, microscopes, illumination systems andlaser systems. In the latter case, a wide range of wavelengths can beused that are more readily absorbed by the metal than by the dielectric.These and other variations upon and modifications to the preferredembodiments are provided for by the present invention, the scope ofwhich is limited only by the following claims.

What is claimed is:
 1. An integrated circuit structure comprising:asubstrate; a submetal dielectric formed over said substrate; a metalinterconnect structure formed over said submetal dielectric, said metalinterconnect structure including at least two metal layers with a topmetal layer having at least two conductors, said conductors having amaximum metal thickness, said conductors being spaced apart by less thansaid maximum metal thickness, said conductors being fused so that theyare shorted; and a passivation layer formed over said metal interconnectstructure, said passivation layer having a minimum passivation thicknessgreater than said maximum metal thickness.